In the electric utility industry, ice accumulation on overhead utility power lines is an ongoing seasonal battle. Even light accumulations of ice on the lines can be enough to cause major interruptions to a utility's power distribution system.
Utility lines typically comprise poles set into the ground with crossarms and other attachment points bolted to the poles to support the line or the wire. The poles commonly are made primarily of wood, as well as fiberglass, steel, concrete or a composite material. After the poles are set in the ground, the wire is pulled from pole to pole and securely fastened to the attachment points. The length of wire that runs from one pole to the next pole is called a span. The wire is pulled in and brought to a pre-calculated tension and a predetermined sag. The sag is the amount of slack in the wire across the span which is most visible at the center point of the span. This slack or sag in the wire can vary from several inches to two feet or more at the center of the span depending on the length of the span and the size of the wire used. As freezing rain causes ice to accumulate on the wire, the added weight pulls the wire down further, and can often cause the wire or the poles and crossarms to break.
Wind driven freezing rain causes the formation of ice on the wire to take an irregular shape, similar to the shape of the wing on an airplane. Due to the aerodynamic nature of this formation, the wind creates lift in the wire. As this form of perpetual motion intensifies, the slack in the wire rises and falls in a very powerful and often violent manner referred to as galloping, which can continue well after the wind has stopped blowing.
When the ice laden line gallops, the slack in the line can rise to a height that is equal to or sometimes greater than the desired sag or lowest center point. Tremendous energy is produced by this condition and can cause severe damage to the section of line, again resulting in broken poles, crossarms, insulators and wire.
A simple distribution single phase line construction consists of two wires. Depending upon the type of construction, one wire at the top of the pole is energized from 2,400 to 7,200 volts or more. The other wire is grounded to earth and called neutral. The neutral can be on a crossarm with the energized phase or lower on the pole. During fair conditions, these two wires form a uniform sag with one another at the center of the span. When ice laden galloping of the lines occurs, however, the slack in both wires is caused to raise and fall at different intervals throughout the span. Depending on where the galloping of each wire occurs, there are times when the two wires make contact with one another, creating a fault that can blow a line disconnect fuse or cause a breaker to open resulting in interruption of service.
At times the two wires do not make contact long enough to interrupt service, but will cause a blinking or flickering of lights. This condition may result in damage to household appliances due to voltage fluctuation, often referred to as a power surge. There are other situations where the galloping wires make contact and burn one or both wires in two. The burned wires then fall to the ground resulting in an interruption of service and pose a threat to public safety.
A three phase distribution line construction consists of three energized phases on a crossarm with the neutral located at a lower point on the pole. In some cases the neutral is located on the crossarm with all three phases. Ice laden galloping wire in three phase systems can be more of a problem than that of single phase. The three energized phases cannot make contact with one another, as well as the neutral.
Three phase voltage is measured in two ways. Phase to phase voltage and phase to ground. A 7,200 volt distribution system would measure 7,200 volts to ground or the neutral and 12,470 volts phase to phase. Therefore, phase to phase contact causes a larger fire and often results in the wire burning to the ground causing costly interruptions of service. Phase to phase contact due to galloping can also cause flickering or blinking lights which can lead to customer appliance damage.
Galloping three phase systems, furthermore, can cause a higher level of damage since there are four wires involved. When the three energized phases and the system neutral gallop at the same time, stresses can exceed the stress rating of the components of the system causing major damage and system failure. These types of damage often cover a large service area affecting many customers, and repair or rebuilding of the overhead electrical system, requires many workers and can take an extended amount of time.
The removal of ice from an entire overhead electric system has been impossible. When the wire breaks or burns down, the ice must be removed from the broken wire or wires. The ice must also be removed from any wires in that span which remain intact. This ice removal is necessary so the repaired wire can be brought back to the original sag as close as possible. Many times the ice needs to be removed in the span or spans on either side of the damaged span because the broken wire slid through the ties at the attachment points on the poles and is hanging much lower than the rest of the wire that remained intact. The ice must be removed from the wire to eliminate the extra weight involved so the two broken ends can be pulled together and repairs made. Ice also must be removed from all wires in the area of the repair made so the sag of the group will match one another as close as possible. This will allow all the wires in the given span to sway with the wind in a more uniform manner thus eliminating the chance for them to contact one another.
If the ice is not removed in these situations, then the repaired wire will end up much higher than the other ice laden wires. In most cases, when the wire broke and fell to the ground much of the ice fell off the two broken tails, thus making them lighter than the remaining ice covered wires. If the wind would create a galloping situation, the higher repaired wire having less ice accumulation would be an easy target for contact from another wire due to the violent galloping action of the ice covered wires and would result in an interruption of service or just burn back down. These repairs are labor intensive and time consuming. It is of utmost importance the repair work is performed correctly the first time.
When temperatures rise above freezing, the ice begins to fall from the wire, which can create further problems. If the temperature should rise above freezing at a quick rate, large lengths of ice fall. When these large lengths of ice fall, the wire sheds large amounts of weight caused by the ice, allowing the wire to slingshot in an upward motion. When this condition exists, the speed of the rising wire and free floating action can lead to contact with another wire or wires resulting in previously stated voltage issues and or system failure. This once again leads to costly and labor intensive repairs and jeopardizes continuity of service to a utility's customer network. The thawing related system interruptions are extremely difficult to control or avoid. Utility workers can only hope the ice layer will thaw and fall at a gradual rate so this condition can be avoided. If downed and broken wire is repaired and the removal of ice on the remaining intact wires are left untouched, the span or spans of wire involved in the repair process are at great risk of thawing related system failures. As previously stated, if one wire was repaired and had ice removed, it would ride at a higher level in the span. Again, the wire would be an easy target to be contacted by a flying wire that had just shed its heavy ice load, leading to system interruption. If the removal of the ice is not performed during the repair process, this leaves the span or spans affected open for failure when thawing occurs. The same work crew can then be required to make repairs in the same span or spans more than just one time.
The removal of ice accumulation from overhead utility power lines for preventing or reducing the foregoing problems can be a very labor intensive process. Utilities are not able to build their lines right along a city street or country road due to right of way issues or extreme added expense to shift construction of a line to a roadway. Therefore, numerous miles of line are built in private right of ways such as unpaved alleys, backyards, farm fields, forests, mountainous terrain, and railroad property. Most of these areas are not accessible to the typical bucket repair truck. Soft soil conditions, extreme mud, building placement on a given property, deep snow or extreme terrain often do not allow access by truck. Owners of these private right of ways also do not want their property severely damaged by heavy utility trucks. There are situations where extreme safety concerns exist and it is necessary to find a way to drive a truck into these areas. The damage to private property in that case cannot be avoided and most utilities will pay for restoration of the damage. When a pole cannot be accessed by a truck, the utility workers are required to physically climb the pole or poles. Climbing a pole is accomplished by using a pair of specially manufactured pole climbers strapped to the worker's boots and a body positioning belt strapped around the waist. The body belt has a long safety strap that is placed around the pole and enables the worker to maintain position on the pole and maneuver as necessary. Climbing a pole requires a great amount of training, practice, skill, and intensive physical exertion. If a crew responds to a downed or broken wire in private right of way that is not accessible by truck, someone has to climb the pole or poles to make repairs.
Wires that remained intact, as indicated above, also require the ice to be removed from them. This is accomplished by either extending fiberglass rods to reach the wire, knocking the ice off the wire, or by using a large cumbersome device called a stringing dolly. A stringing dolly consists of a large roller attached inside an aluminum framework with a trap door that can be opened for the placement of wire inside the frame to ride on the roller. This trap door can be locked in the closed position to avoid having the wire fall out of the roller. The line must be de-energized in order to remove the ice with the stringing dolly, which puts customers who still had service out of service until repairs are made.
A worker atop the pole places the dolly on the ice laden wire and then ties a long rope to the dolly. Another worker on the ground then takes the rope and pulls down with force. While pulling down on the rope and dolly, the worker then walks to the next pole. Pulling down with force and dragging the dolly at the same time, peels the ice accumulation from the wire. When the worker on the ground arrives at the next pole in the line, a return trip is required back to the pole of origin. Upon arrival at the pole of origin, the worker atop the pole removes the dolly and places it on another wire if needed. This removal process is performed on all the wires in the affected span or spans. This process requires a great amount of physical energy and stamina in both climbing the poles and downward pulling of the dolly. In addition, to effect removal of the ice, it is necessary for the worker to walk nearly directly under the dolly to keep enough downward pulling force on the rope to peel off the ice. At times, large lengths of ice can fall during this process and pose a safety hazard to the worker below. The process requires a number of workers, and two or three crew members on the ground often trade off to avoid exhaustion. Due to the intense labor involved, this process is commonly referred to as “running the heater”. A worker will expel enough energy to raise body temperature and produce large amounts of sweat creating issues of staying warm in the cold conditions after the worker has rested and cooled off.
Although ice storms are perennial problems in the maintenance of utility lines, the physical task of removing ice from utility power lines has remained unchanged for many years, notwithstanding the difficulty, worker fatigue, damage to surrounding areas, and safety concerns. Hence, a great need has existed for an improved system for removing ice from utility power lines.